1. Field of the Invention
The invention relates to high recording density magnetic tape recording. The invention further relates to cycle slip detection and correction.
2. Background Art
When migrating into high recording density magnetic tape recording, cycle slips are considered to be a major limiting factor in the performance of the read/write channel as they cause long bursts of bit errors. Cycle slip is an event where the clock gets out of synchronization with the data, causing part or all of the bits in a data sector to be in error until the end of the data sector. FIG. 1 illustrates an original bit sequence at 10 and a detected bit sequence at 12 suffering from a cycle slip which occurs at 14. From FIG. 1, it is shown that all of the bits after a cycle slip are shifted one bit to the right in the detected bit sequence 12. Since all the bits after the occurrence of a cycle slip are in error, it is formidable to use error correction coding (ECC) to correctly recover the user data from cycle slips. Hence, it is crucial to design special methods to reduce the negative impacts of cycle slips.
Clearly, any signal interruption or failure of the PLL loop to correctly follow the incoming readback signal will cause cycle slips. Similarly, the failure to correctly detect and count an incoming waveform due to poor signal-to-noise ratio will also result in cycle slips. Further, dropouts can easily cause cycle slips because dropouts result in significant shifting of signal peak locations and this peak shift may generate cycle slips. Still further, the longitudinal vibration (tape path resonance) problem that exists in tape recording systems triggers very frequent cycle slips.
Cycle slip is a serious problem that prevents reliable read/write channel performance at high recording densities. The cycle slip problem can not be handled by the standard ECC.
FIG. 2 illustrates the major components in a tape write channel. As shown, sector data 20 is received from the user, passes through ECC encoder 22, passes through modulation encoder 24, and then is handled by the write hardware 26. The read hardware is indicated at 28, and passes read data to the read channel. The tape media is shown at 30.
FIG. 3 illustrates the major components in a tape read channel. As shown, variable gain amplifier 40 with automatic gain control 42 receives a signal from the pre-amplifier of the read hardware. The signal passes through asymmetry correction block 44, having automatic correction control 46, and through low pass filter 48, to sampler 50. The sampled data passes through equalizer 52 and then to Viterbi detector 54. Viterbi detector 54 detects the likely sequence of bits, and as appropriate, indicates to timing recovery block 56 that there is a timing problem with the detected signal. The bit sequence detected by the Viterbi detector 54 passes through modulation decoder 60 and ECC decoder 62 to result in the retrieved data.
Background information may be found in U.S. Pub. No. 2009/0019335.
For the foregoing reasons, there is a need to detect cycle slips and correct erroneous data sectors corrupted by cycle slips.